1. Interesting Yet Hitherto Unknown World of Molecular Anions
Vaibhav S. Prabhudesai
TIFR Mumbai

2. Molecular Dynamics and Control using Photons and Electrons (MDCPE)
Acknowledgement
Molecular Dynamics and Control using Photons and Electrons (MDCPE)
E. Krishnakumar (Now at RRI)
Krishnendu Gope (Now at HUJI)
Sramana Kundu (Now at Georgia Tech)
Vishvesh Tadsare
Daly Davis (Now at BARC)
Samata Gokhale
Technical Help
Yogesh Upalekar
Satej Tare
External Collaborators
Nigel Mason
Open University, UK
Y. Sajeev
BARC, Mumbai
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3. Relevance of Negative Ion Studies
Planetary atmospheres – Environment
Laboratory and Astrophysical plasmas
Fusion research, Gas lasers
Semiconductor electronics
High power switches, Dielectrics
Analytical techniques – Mass spectrometry
Isotope separation, Neutral beam generation
Radiation Physics and Dosimetry
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5. O-, O2-, O3-, CO2-, NOx-, etc.

6. Etching (IC’s, cleaning):. CF4, C2F6, C3F8, SF6, O2… Deposition:
Etching (IC’s, cleaning): CF4, C2F6, C3F8, SF6, O2… Deposition: CH4, SiH4, NH3

7. Excimer media: Ar, Kr, Xe + F2, Cl2 Discharge lamps

8. Gaseous Dielectrics

9. Radiation Damage 5 x 10 4 electrons per MeV energy of the radiation
Electron energy varying upto ~ 20 eV

10. A sting in the tail of electron tracks

11. Negative Ion ResonancesX
E. A.
(Electron Affinity) X–
Resonance
Conventional optical spectroscopy fails in studying these resonances
Studied through electron collisions
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12. Basic electron collision processes in molecules
Elastic scattering AB + e AB + e -
Inelastic scattering
Excitation AB + e AB* + e -
Ionization AB + e AB+ + 2e -
Dissociative ionization AB + e A + B + + 2e -
Dissociation AB + e A + B + e -
Dissociative excitation AB + e A + B* + e -
Polar dissociation AB + e AB* + e -
A + + B -
Dissociative electron attachment AB + e (AB -) * A + B -
Bond breaking by catalytic electron A + B + e -
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Most effective way of converting kinetic energy into chemical energy

13. Schematic of Electron Molecule Collision for DEA
Temporary negative ion (TNI)
e -
((A-B)-)*
A-B
Dissociation
A B-
Auto-detachment
(A-B)*
+ e -
Resonant Scattering
(Electron spectroscopy)
Dissociative Electron Attachment
(Negative ion mass spectrometry)
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14. DEA Cross Section Prabhudesai et al Phys. Rev. Lett. 95, 143202 (2005)
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Prabhudesai et al Phys. Rev. Lett. 95, (2005)

15. Quantum Coherence in Dissociative Electron Attachment
Dissociative electron attachment to molecular hydrogen
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16. Dissociative Electron Attachment to H2
Krishnakumar et al. PRL (2011)
For third resonance at 14 eV
𝐻 2 +𝑒 → 𝐻 − 𝑆 +𝐻(2𝑙)
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17. Angular Distribution of Fragment Ionsq e- A B-
A-B
(A-B)-
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18. Angular Distributions in Pictures
Expected angular distributions homonuclear diatom
H2 ground state is Σ g +
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19. The 4 eV Resonance e-
2Su+
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20. Potential Energy Curves
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21. The 14 eV Resonance
14.5 eV
15 eV
15.5 eV e-
4.5 eV
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22. Momentum Images H2
14.5 eV
15 eV
15.5 eV D2
14.0 eV e-
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23. How can the symmetry be broken ?
𝐻 Σ 𝑔 𝑒 − 𝐸 → 𝐻 2 − ∗ Σ 𝑢 + 𝑝 Σ 𝑔 + 𝑠
→ 𝐻 − 𝑆 +𝐻 𝑛=2
Angle differential cross section
For Σ 𝑔 + +𝑠−𝑤𝑎𝑣𝑒→ 2 Σ 𝑔 +
For Σ 𝑔 + +𝑝−𝑤𝑎𝑣𝑒→ 2 Σ 𝑢 +
𝐼 𝜃 = 𝜎 𝐷𝐸𝐴 𝑔
𝐼 𝜃 = 𝜎 𝐷𝐸𝐴 𝑢 𝑐𝑜𝑠 2 (𝜃)
For coherent excitation of the two resonances
𝐼 𝜃 = 𝜎 𝐷𝐸𝐴 𝑔 𝜎 𝐷𝐸𝐴 𝑢 𝑐𝑜𝑠 2 𝜃 𝑐𝑜𝑠𝛿 𝑐𝑜𝑠𝜃 𝜎 𝐷𝐸𝐴 𝑔 𝜎 𝐷𝐸𝐴 𝑢
δ is the phase difference between the two channels.
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24. Contrast of the Interference Pattern
The relative phase between the two quantum paths
Intrinsic phase difference between absorbed partial wave for the incoming electron
Potential scattering before capture of the electron
With common start and end point the wavepackets following two different potential energy curves
Relative amplitudes from the paths
Capture cross section for each resonance
Dissociation time and Corresponding lifetime of ions against autodetachment
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25. Interfering Quantum Paths
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26. Estimate of the Symmetry BreakingD2
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27. Breaking Inversion Symmetry
Coherent excitation to a state by more than one path
Resulting interference between the paths give control over the out put
If the paths are of opposite parities
Results in inversion symmetry breaking of the dissociation
Wu et al, Phys Rev A, 87, (2013)
Kling et al, Science, 312, 246 (2006)

28. Some interesting points
The attaching electron is from an incoherent beam
Multiple channels of attachment through different angular momentum transfers
The coherence in the superposition of the anion states is through coherence in the partial waves
Will the lifetime of the shorter lived resonance affect the lifetime of the longer lived resonance?
Ideal system to explore the quantum effect as all three electrons and 2 nuclei participate in the process!
Decay of superposition of two resonances
Frishman & Shapiro, PRL 87, (2001)
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29. What about the electron ejection channel?
Weingartshofer et al, Phys Rev A, 2, 294 (1970)
Ehrhardt and Weingartshofer, Z Phys, 226, 28 (1969)

30. Site Selectivity in Dissociative Electron Attachment
Dissociative electron attachment to organic molecules
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31. Origin Of Hydride Ion O C H
CH3COOH
CH3COOD O C H D

32. Site Selectivity in Dissociative Electron Attachment (DEA)
Observation of bond orientation and site selectivity in DEA
O-H vs C-H bonds
N-H vs C-H bonds
The resonances observed in H- channels for organic molecules are found to follow the precursors of the functional groups
O-H to follow H2O
N-H to follow NH3
S-H to follow H2S
C-H to follow CH4
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33. Hˉ from Amine H- / CH4 H- / NH3
CH3CH2CH2NH2
Prabhudesai et al. PRL 95, (2005)
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34. Site Selectivity In DEA Beyond Threshold
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Ptasin´ska et. al. Ang. Chem. Int. Ed. 44, 1647 (2005)

35. For Thiol Group Thioacetic acid (CH3COSH) Hydrogen Sulphide (H2S)
N. B. Ram et al PCCP 13, (2011)
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36. Free Electron as a Catalyst
Bond breaking by catalytic electron in formic acid
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37. Free electron can act as a catalyst is theoretically predicted earlier
Reduces activation barrier
Does not modify the standard Gibbs free energy change in the reaction
It is retrieved back at the end of the reaction
Free electron can act as a catalyst is theoretically predicted earlier
IUPAC Gold Book
D. Davis et al., Angew. Chem., Int. Ed., 2011, 50, 4119
D. Davis et al., Angew. Chem., Int. Ed., 2012, 51, 8003

38. Formic Acid Decomposition follows Catalytic pathway:
Dehydrogenation on transition metal surface
𝐻𝐶𝑂𝑂𝐻 → 𝐶𝑂 2 + 𝐻 2
Non-Catalytic pathway:
Dehydration by thermal activation
𝐻𝐶𝑂𝑂𝐻 → 𝐶𝑂 + 𝐻 2 𝑂
Can we mimic catalytic route directly by free electrons?

39. Electron Irradiation of Formic Acid Film

40. CO2 formation from HCOOH

41. Gas Phase
Condensed Phase
HCOO–
O–/ OH– H–

42. Mechanism of catalytic activity

43. Can we control chemical reactions using electrons?
To Summarise
There are many unexplored facets of anion dynamics
Quantum coherence in attachment of incoherent electron beam
Site selectivity in dissociation of organic molecules
Role in catalytic action of free electron
Next
Can we control chemical reactions using electrons?
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44. Scanning Tunneling Microscope
Binnig and Rohrer: Nobel Prize 1986

45. Can we see molecules using electrons?
Pentacene

46. Controlling Molecular Motion With Scanning Tunneling Microscope

47. Single Molecule Chemistry!

48. It can change left handed molecule to right handed molecule

49. Play with the molecules!

50. Thank you